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06 June 2010

BOBGRAM7 issued 6 June 2010

WEATHERGRAMYOTREPSIssued 6 June 2010Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are fromthe patterned world of weather maps, so please fine-tune to your place.Dates are in UTC unless otherwise stated.

Whoops - I see that last week's email heading was dated 30 July 2010instead of 30 May 2010.

TROPICSSouth Pacific Convergence zone SPCZ is still gradually reforming along10S, from Papua New Guinea to east of Solomons to Tuvalu/Tokelau andextending to Samoa at times, also extending south between Vanuatu andFiji at times. During this week it seems that there will be moreconvective activity building in the Coral Sea, and this may culminate ina trough/low forming west of New Caledonia by around 19 June (end ofNEXT week).

There has also been more rain clouds than normal in a zone from justsouth of New Caledonia onto northern NZ . This zone isn't part of SPCZ,rather it is a combination of the northern end of mid-latitude frontalsystems + jetstream intensification + a large upper low stuck in thewestern Tasman Sea and developing a series of surface lows to throw atNZ, like a bowler player cricket. This "low index" pattern started inmid May just in time to harass this years Auckland to Fiji race. Itseems to me that the pattern is about to change this week to the morefamiliar "Roaring 40s" pattern.

SUBTROPICAL RIDGE: STRThe STR has been absent over NZ since mid May. There have been thinridges squeezing along 20 to 30S - north of the normal latitude, makingfor lighter than normal trade winds between Fiji and French Polynesia.This is about to change. The next STR is over central New South Walestoday (sun 6 June) and should wander east between 30 and 40S, crossingNZ on Wednesday 9 June, and then settle between 25 and 35 South as itwanders east of NZ.

The following STR should be a large HIGH in the Aussie Bight on Fri 11June. As it crosses Bass Strait on the weekend of 12 /13 June it shouldshovel polar air onto southern NZ producing a solid dump of snow there.Models indicate this High will not be able to get into the Tasman sea at40S , so it should go N across eastern OZ on 14/15/16 June and thenmove east along 30 to 35S - across northern Tasman sea and north of NZon 17/18/19 June--- with enhanced easterlies on its northern side andenhanced westerlies on its southern side --- the "Roaring 40s" will beback.

TASMAN/NZLow crossing central NZ on Monday should go southeast across Chathams onTuesday. There is a sting in its tail, as one more squally trough withW to SW winds is expected to be crossing northern NZ on Tuesday as well.

Light winds with a ridge on Wednesday.

Trough/front is expected to move from Tasman Sea across NZ on Thursdayand early Friday. The SW winds following this trough should reachNorthland Friday morning, producing a reasonable looking window forsailing northwards.

Additionally, there is a brief window late Tuesday and early Wednesday,with the minor impediment of some pre-frontal northerly winds onThursday and - note well- following the Friday front - there isexpected to be a period of heavy westerly swell over the seas north ofNorthland.

I'm gearing up now to attend to MetService display at Fieldays from Tue15 to Sat 19 June. There should be a briefer weathergrams issued Sun 13June, then I'll mostly be unavailable until 19 June,